1. Field of the Invention
The present invention relates to an electrolyte for a lithium battery and a lithium battery comprising the same, and more particularly, to an electrolyte for a lithium battery capable of improving overcharge inhibition properties and electrochemical characteristics, and a lithium secondary battery comprising the same.
2. Description of the Related Art
The use of portable electronic instruments is increasing as a size of electronic equipment becomes smaller and a weight becomes lighter due to developments in high-tech electronic industries. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having a high energy density for use as a power source in these portable electronic instruments. Such a lithium secondary battery, having an average discharge potential of 3.7 V (i.e., a battery having substantially a 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as cellular telephones, notebook computers, camcorders, and the like (i.e., the “3C” devices).
Also, there has been extensive research on batteries with effective safety characteristics, such as, for example, preventing overcharge. When a battery is overcharged, an excess of lithium ions is deposited on a positive electrode, and an excess of lithium ions is also inserted into a negative electrode, rendering the positive and negative electrodes thermally unstable. An eruptive explosion occurs from a decomposition of the electrolytic organic solvent, and the thermal runaway that occurs causes serious problems of battery safety.
To overcome the above problems, it has been suggested that an aromatic compound such as an oxidation-reduction additive agent (“redox shuttle”) be added to the electrolyte. For example, U.S. Pat. No. 5,709,968 discloses a non-aqueous lithium ion secondary battery to prevent thermal runaway resulting from overcharge current by using a benzene compound such as 2,4-difluoroanisole. U.S. Pat. No. 5,879,834 discloses a method for improving battery safety by using a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, furan, and the like, which is polymerized electrochemically to increase the internal resistance of a battery during unusual overvoltage conditions. Such redox shuttle additives increase the temperature inside the battery quickly due to heat produced by the oxidation-reduction reaction, and close pores of a separator through quick and uniform fusion of the separator to inhibit an overcharge reaction. The polymerization reaction of these redox shuttle additives consumes the overcharge current to improve battery safety.
However, the polymerization of these redox shuttle additives cannot sufficiently eliminate the overcharge current. In addition, decomposition of the additives causes gas generation inside the battery, and thus a certain plane of the battery swells. Therefore, improvements in the safety of the battery are limited when using the redox shuttle additives, so that there is a need for research on electrolytes to prevent overcharge and to ensure safety.